专栏主页：计算机专业基础知识总结（适用于期末复习考研刷题求职面试）系列文章https://blog.csdn.net/seeker1994/category_12585732.html

题目描述

请实现一个函数用来判断字符串是否表示数值（包括整数和小数）。例如，字符串"+100","5e2","-123","3.1416"和"-1E-16"都表示数值。 但是"12e","1a3.14","1.2.3","+-5"和"12e+4.3"都不是。

题解报告

编译原理状态机

# include <iostream>
# include <string>
using namespace std;
bool IsNum(char ch){
    if(ch<'0'||ch>'9') 
		return false;
    else 
		return true;
}
bool isNumeric(char* str){
    int i = 0;
    if(!(str[i] == '+' || str[i] == '-' || IsNum(str[i])) )
		return false;
	else{
		i++;
        while(IsNum(str[i]) && str[i] != '\0')
			i++; //如果是数字一直往后找
		if(str[i] == '\0'){
			return true;
		} 
        else if(str[i] == '.'){
        	i++;
        	if(str[i] == '\0')
				return true;
            else if(IsNum(str[i])){
                while(str[++i]!='\0' && IsNum(str[i]));
                if(str[i]=='e'||str[i]=='E'){
                    i++;
                    if(str[i]=='+' || str[i]=='-' || IsNum(str[i])){
                        while(str[++i]!='\0' && IsNum(str[i]));
                        if(str[i]=='\0')
							return true;
                        else
							return false;
                    }else
						return false;
                }else if(str[i]=='\0'){
                	return true;
				}else{
					return false;
				}		
            }else{
            	return false;
			}
        }else if(str[i]=='e'||str[i]=='E'){
            i++;
            if(str[i]=='+' || str[i]=='-' || IsNum(str[i])){
                while(str[++i]!='\0' && IsNum(str[i]));
                if(str[i]=='\0') return true;
                else return false;
            }else return false;
        }	
        else{
        	return false; 
		}	         
    }
}
int main(){
	char str[] = "1.25";
	if(isNumeric(str)){
		cout<<"YES"<<endl; 
	}else{
		cout<<"NO"<<endl;
	}
	return 0;
}
优化后的代码，使用状态表:
# include <iostream>
# include <string>
using namespace std;
char arr[10] = "+-n.ne+-n";
int turn[10][9] = {
   //+  -  n  .  n  e  +  -  n
    {1, 1, 1, 0, 0, 0, 0, 0, 0},    // # start
    {0, 0, 1, 1, 0, 0, 0, 0, 0},    // +
    {0, 0, 1, 1, 0, 0, 0, 0, 0},    // -
    {0, 0, 1, 1, 0, 1, 0, 0, 0},    // n
    {0, 0, 0, 0, 1, 0, 0, 0, 0},    // .
    {0, 0, 0, 0, 1, 1, 0, 0, 0},    // n
    {0, 0, 0, 0, 0, 0, 1, 1, 1},    // e
    {0, 0, 0, 0, 0, 0, 0, 0, 1},    // +
    {0, 0, 0, 0, 0, 0, 0, 0, 1},    // -
    {0, 0, 0, 0, 0, 0, 0, 0, 1}     // n
};
bool isNumeric(char* string) {
    int cur = 0,i,j;
    for(i = 0; string[i]; i++) {
        for(j = 0; j < 9; j++) {
            if(turn[cur][j]) {//状态转移 
            	//状态转移 
                if(('0' <= string[i] && string[i] <= '9' && arr[j] == 'n') ||
                    (string[i] == 'E' && arr[j] == 'e') ||
                    string[i] == arr[j]) {
                    cur = j + 1;
                    break;
                }
            }
        }
        if(j == 9)
			return false;
    }
    if(cur == 3 || cur == 4 || cur == 5 || cur == 9)
       return true;
    return false;
}
int main(){
	char str[] = "1.25";
	if(isNumeric(str)){
		cout<<"YES"<<endl; 
	}else{
		cout<<"NO"<<endl;
	}
	return 0;
}

#1正则
public boolean isNumeric(char[] str) {
    String string = String.valueOf(str);
    return string.matches("[\\+-]?[0-9]*(\\.[0-9]*)?([eE][\\+-]?[0-9]+)?");
}
//标记法
bool isNumeric(char* str) {
    // 标记符号、小数点、e是否出现过
    bool sign = false, decimal = false, hasE = false;
    for (int i = 0; i < strlen(str); i++) {
        if (str[i] == 'e' || str[i] == 'E') {
            if (i == strlen(str)-1) return false; // e后面一定要接数字
            if (hasE) return false;  // 不能同时存在两个e
            hasE = true;
        } else if (str[i] == '+' || str[i] == '-') {
            // 第二次出现+-符号，则必须紧接在e之后
            if (sign && str[i-1] != 'e' && str[i-1] != 'E') return false;
            // 第一次出现+-符号，且不是在字符串开头，则也必须紧接在e之后
            if (!sign && i > 0 && str[i-1] != 'e' && str[i-1] != 'E') return false;
            sign = true;
        } else if (str[i] == '.') {
          // e后面不能接小数点，小数点不能出现两次
            if (hasE || decimal) return false;
            decimal = true;
        } else if (str[i] < '0' || str[i] > '9') //不合法字符
            return false;
    }
    return true;
}